The present invention relates generally to optical networking, and more particularly, to a tunable bidirectional multiplexer/demultiplexer for an optical transmission system in which added and dropped signals follow a common optical path through a wavelength-selective switch.
The development of optical fiber communication technologies has enabled exponential growth in the capacity of backbone networks. Commercially deployed dense-wavelength-division multiplexing (DWDM) optical communication systems can now carry over 1 Tbps in a single fiber, and experimental applications have demonstrated much greater capacities.
Fiber optic distribution networks are becoming increasingly important for the provision of high bandwidth data links to commercial and residential locations. Such systems employ optical data transmitters and receivers (“transceivers”) throughout the fiber optic distribution network. These transceivers generate optical signals for optical transmission over optical fibers and receive optical signals from the fibers for processing or forwarding. In some systems (typically those found in networks carrying asymmetric traffic, such as CATV systems) the transmitters (for generating optical signals) and the receivers might not be integrated into a single unit.
In a traditional WDM system, a single optical fiber simultaneously communicates a plurality of different communication channels in light of different wavelengths. Generally, each communication channel has an assigned central wavelength and channel spacing is defined for the network. DWDM network standards have optical channels with frequency separations of 25, 50 and 100 GHZ.
As optical technology has become more sophisticated, additional network functionality has migrated from the electronic domain to the optical domain. In the past, transmission systems were all point-to-point. All wavelengths on a system were transmitted between the same two nodes. To reach their final destination, signals were routed from one point-to-point transmission to another, with an optical-electrical-optical conversion at each node along the way. The optical transmission systems were used for transmission only. To dynamically redirect a signal's path it was converted to an electrical signal, and switching was performed in the electrical domain.
Many modern commercial optical systems have the ability to add/drop wavelengths from a line system at a node, while other wavelengths pass through the node on an express path. When an add/drop multiplexer can be dynamically adjusted it is known as a reconfigurable-optical-add-drop multiplexer (ROADM). Advanced ring networks with ROADM's are being widely deployed. These allow a transmission system to serve multiple nodes without requiring that all wavelengths be regenerated at each node.
In mesh networks, where many nodes may be connected to three or more other nodes, an all-optical photonic-cross-connect (PXC) can provide similar functionality.
Another recent innovation in optical networks is the availability of tunable transmitters. These transmitters have a tunable laser, so that the signals they transmit can be carried on any of the system's wavelengths. This enables a transceiver to be used for any wavelength channel. Currently, this makes it easier to provision new wavelengths, and maintain spare parts for the network's transceivers. The receiver portion of the transceiver can convert any wavelength from an optical to an electrical signal, but it must be preceded by an optical filter, so that only one wavelength channel reaches the optical receiver.
If all the elements of the network were tunable, then additional benefits could be achieved. Wavelengths could be dynamically routed throughout the network. Initially this would be used to provide rapid provisioning, but it could eventually be used to provide protection switching, or even to provide novel services that require very rapid wavelength switching. With tunable transceivers and ROADM's already present in deployed networks, the last component of commercially available networks to be tunable is the wavelength multiplexer and demultiplexer. The wavelength multiplexer lies between the tunable transmitter(s) and the add port of the line system. The wavelength demultiplexer lies between the receiver(s) and the drop port of the line system. The add/drop port may be located at the line system's end terminal, or at a ROADM or PXC. A typical wavelength multiplexer or demultiplexer has wavelength-specific ports for the connections to the transceivers, so that once the transmitter is connected to a port of the multiplexer or demultiplexer it must be tuned to that port's wavelength in order to transmit the signal onto the line system, and the receiver can only detect the signal at the receive port's assigned wavelength.
Most WDM components and systems use separate fibers for transmission in each direction, e.g. signals going from East to West travel on one fiber, while the signals going from West to East travel on another fiber. We shall refer to systems and components which transmit signals in only one direction on each fiber as unidirectional, while those which carry signals in both directions on a single fiber are referred to as bidirectional.
Tunable unidirectional wavelength multiplexers and demultiplexers for adding and dropping a wavelength channel to and from a transmission system with a node are known in the art. It is also known that these tunable multiplexers may comprise wavelength-selective switches (WSSs) on the multiplexer side to multiplex a plurality of wavelength channels that are being added to the optical transmission system. Tunable filters or an additional WSS can be utilized to demultiplex wavelength channels that are dropped from the optical transmission system to the local terminal. WSSs are commercially available devices that dynamically route signals from the input port(s) to the output port(s) based on the wavelength of the signal, in response to control signals that set the WSS's connection state. In unidirectional multiplexers and demultiplexers, separate optical components are used to multiplex and demultiplex the signals.